In DC-DC power supplies, optimization of the power/control MOSFET (metal oxide semiconductor field effect transistor) requires minimization of both conduction and switching losses. For example, LDMOS (laterally diffused MOSFET) devices have historically been used in RF (radio frequency) applications that require very low switching loss at high frequency.
The semiconductor industry defines ruggedness of a power MOSFET as the capability to withstand avalanche currents when subjected to unclamped inductive switching (UIS). For power switching applications, inductance cannot be avoided in every electrical circuit. That is, in a UIS event, inductance in a circuit that is switched off through a power MOSFET will continue to push current through the power MOSFET. This results in high voltages present across the transistor, which in turn leads to failure of the power MOSFET, such as, avalanche breakdown and high temperatures. As such, this unclamped inductive switching event remains one of the most critical challenges to power MOSFET ruggedness.
One of the key properties of inductance is it will absorb energy from the circuit during a turn-on process, and release the energy into the circuit during a turn-off process. For example, whenever current through an inductance is quickly turned off, the magnetic field inducts a counter electromagnetic force (EMF) that can build up surprisingly high potentials across the corresponding switch. When this UIS event happens, since there is no clamp device to take over the energy stored in the inductance, such huge energy has to be consumed by the power MOSFET device or otherwise fail. That is, when transistors are used as switches, the full buildup of this inducted potential may far exceed the rated breakdown voltage of the transistor, or result in an instantaneous chip temperature reaching a critical value. In either case, the power MOSFET in an uncontrolled UIS event will experience catastrophic failure.
As such, the power MOSFET device must survive and function as usual again after a UIS event is over. For faster power switching, such as within an RF application, UIS immunity becomes more challenging and important.